Conventional turbine engines include three main parts, a compressor, a combustor, and a turbine. Fuel is mixed with compressed air from the compressor and burned in the combustor. The resulting flow of combustion products out of the combustor subsequently drives the turbine. Typically, the fuel and air may be mixed in a fuel pre-mixer, before being injected into the combustor. Alternatively, the fuel and air may be directly injected into the combustor without premixing. This may result in a high temperature combustion, leading to the production of considerable volumes of NO and NO2, generally referred to as NOx. Premixing the fuel and air prior to combustion to maintain a lean fuel-air ratio produces lower reaction zone temperatures and thus lowers NOx emissions.
However, if the fuel-air mixture is too lean, it may result in incomplete combustion leading to excessive emissions of carbon monoxide (CO) and unburned hydrocarbon (UHC). Additionally, low fuel-air ratio may also result in flame blowout requiring the engine to be started all over again. To minimize CO and UHC emissions, the reaction zones in the combustor of turbines engines may have a fuel-air ratio sufficient enough to avoid blowout but lean enough to significantly reduce NOx emissions. To balance the conflicting needs for reduced CO, UHC, and NOx emissions, extremely precise control is required over the fuel-air mixture in the reaction zones of the combustor in an industrial turbine engine.
Operation at low bulk fuel/air ratio, near a lean extinction limit, is particularly difficult at reduced load. That is, during off-peak hours operating a generator at full output is not practical. Any energy produced over demand that is not otherwise sold is wasted. Accordingly, balancing low output with lean operation while mainlining emissions compliance is difficult. In order to address this problem the turbine engine is operated at a piloted-premix in which some 10 to 20% of the fuel is injected directly into the reaction zone and burns as a high temperature diffusion flame. This provides good stability and combustion efficiency, but NOx levels are out-of-compliance. Thus, the turbine engine is alternately operated in an out of compliance state and in compliances state to maintain average emissions output in compliance.
In addition to the above, restarting a turbine combined cycle generator that was shut off is a lengthy process that may take an hour or more before full output is achieved. This lost time can be quite costly for an energy producer. Moreover, a generator that is shut off is not available in the event that additional output is unexpectedly needed during a low demand period. In addition, starting and stopping a generator impacts the durability and life of power system components. Frequent starts and stops will have a detrimental impact on engine reliability and trigger a need for more frequent maintenance cycles thus increasing operational and maintenance costs.
Given the drawbacks associated with stopping the gas or combined cycle turbine engine, energy producers prefer to turn down or park the engine during off peak hours to minimize the fuel burned while maintaining the ability to respond to an unplanned increase in load. Parking the turbine engine at a point that allows a quick return to full power, while also remaining emission compliant, can be difficult for the reasons outlined above. Therefore, when parking a turbine engine, the engine is operated at a specific part load condition with brief periods of out-of-compliance operation. While effective at maintaining a turbine engine within emission compliance, achievable part load conditions are still high, in the range of 40% of normal output, and thus can represent substantial operational inefficiencies.
In addition to the above, an important over-arching constraint that represents a significant initial barrier and steady, day-to-day struggle in successfully addressing all emissions, reliability and operational flexibility requirements of a turbine engine is the variation inherent in any ‘real-world’ power plant context. Performance of a lean, premixed combustion system may be impacted by minute changes in external variables. Variation in individual fuel circuit flow (fractions of 1% of the total), night/day and seasonal variations in ambient temperature and relative humidity, site location and elevation, and incremental (a few percent by volume) changes in fuel gas composition, as well as power system load, can impact combustion system performance.
Moreover, in turbine engines having a plurality of combustors, such as in a can-annular architecture, it is important that the fuel-air ratio in each combustor of the can-annular design should be substantially the same or adjusted as appropriate for the system design. For example, a constant fuel-air mixture in each combustor allows the mixture to be maintained at the lean ratio that best reduces CO, UHC, and NOx emissions. In addition, uniform fuel-air ratios among the different combustors ensure a uniform distribution of temperature among the combustors of a turbine engine. A uniform distribution of temperature and pressure reduces the thermal and mechanical stresses on the combustion, turbine, and other hot stream components of the turbine engine. A reduction in these stresses prolongs the operational lives of the different combustors and turbine parts. Peak hot gas temperature in some amongst the combustors increases thermal stresses and reduces the strength of materials in the hotter high fuel-air ratio chambers and turbine parts immediately downstream of those chambers.
However, achieving truly uniform temperature and pressure distribution in the different combustors in a can-annular architecture has traditionally been found to be difficult. This may be due to the inherent variations existing between the similar combustors forming the can-annular architecture. These variations arise out of the tolerances involved in the manufacturing, installation, and assembly of each of the combustors with other components the turbine engine. These variations in the components of the combustors and their assembly can perturb the incoming air flow into the combustors. The different perturbations can cause different non-uniformities to the flow in the different combustors. Thus, the fuel-air ratio is affected differently in each combustor. Variations in the air flow in each combustor can make it difficult to maintain constant fuel-air ratios in all the combustors. Thus, to maintain uniform fuel-air ratios in the different combustors, the airflow in the different combustors need to be controlled. The current literature manages airflow balance by precise control of circuit effective flow areas, such as requiring close manufacturing and assembly tolerances. In practice, the level of manufacturing precision and the functional testing required can be costly. Further, there are high thermal and mechanical operating loads on the turbine engine that result in deformation, creep and loss of dimensional control.
Accordingly, there is a need for methods and systems for controlling a combustor in turbine engines. There is a further need for controlling the air flow in a combustor in turbine engines. There is yet a further need for dynamic balancing of the air flow to a combustor that can account for the structural and dimensional changes in the components of the turbine engine over time.